Although DTC patients receive standardized treatment and the 10-year survival rate is more than 90%, more than 60% of DTC patients still develop RAIR-DTC due to dedifferentiated DTC cells and loss of iodine-concentrating ability. Unlike DTC, the survival time of RAIR-DTC patients is significantly shorter, with an average survival time of three to five years and a 10-year survival rate of only 10%, which has become the focus of clinical attention (4).
The NIS protein expression is closely related to the curative effect of 131I. BRAF V600E mutation has been reported to affect the localization and expression level of NIS protein, thus causing a poor therapeutic effect of RAI (22). BRAF V600E mutation occurs in about 45% (27–87%) of DTC patients, closely related to DTC initiation, development, metastasis, iodine uptake, and poor prognosis (23). We found that TESC expression was significantly upregulated in DTC tumor tissues and positively correlated with BRAF V600E mutation through deep TCGA data mining. Single factor-related analysis of clinicopathological characteristics revealed histological types, ETE, LNM, N stage, BRAF V600E mutation, and TDS were all significantly associated with TESC expression (Table 1). Multivariate analysis found that high TESC expression was an independent factor for LNM (Tables 2 and 3). Moreover, TESC high expression was also found in DTC samples of our center, associated with ETE and BRAF V600E mutations (Table 4). Thus, TESC is considered a poor prognostic factor for metastasis in DTC.
As we have shown that increased TESC expression is an independent factor for LNM, we hypothesized that it promotes migration and DTC cell invasion, which is related to EMT features. Epithelial cell-derived tumor cells acquire the ability to move and invade after EMT and then metastasize to distal through blood and lymphatic circulation. This process is abnormally activated during DTC development (24). DTC cells have been reported to exhibit an active EMT process, characterized by loose epithelial features, polarity, and cohesion, decreased epithelial markers expression and increased mesenchymal markers expression compared with normal thyroid cells (25, 26). Typical EMT shows increased mesenchymal markers expressions such as N-cadherin, vimentin, fibronectin, and a-smooth muscle actin (a-SMA). TESC knockdown in IHH-4 (BRAF V600E mutation) and TPC-1 (BRAF V600E wild type) cells significantly inhibited migration, and invasion (Figs. 2F, and G). They downregulated the EMT pathway markers Vimentin and N-cad, while increased E-cad (Figs. 2D and E). Moreover, it has been reported that in non-small cell lung cancer (NSCLC), liver cancer, and thyroid cancer, an increase in TESC enhanced the tumor cells' invasiveness (8, 9, 12, 13). TESC overexpression and PTEN silencing observably promoted the viability, colony formation, migration, and invasion of DTC cells (13). Furthermore, Zou et al. demonstrated that TESC promoted the PTMC progression by regulating the c-Fos expression related to ERK signaling pathway and detected the expression of EMT pathway-related indicators (E-cad, N-cad, Vimentin, Snail) (12), which are consistent with our results, though an ATC cell line was used.
DTC cells commonly retain the expression and function of NIS, which is the physiological basis of radioactive iodine (131I) uptake. The dedifferentiation of DTC into poorly or dedifferentiated DTC cause DTC cells to lose their ability to uptake radioactive iodine due to downregulation and dysfunction of NIS (27). Moreover, BRAF V600E mutation activated the ERK1/2 signaling pathway contributing to DTC dedifferentiation (28). Recently, it has been reported that TESC/FOS/ERK1/2 signaling axis was involved in the growth and metastasis of PTC cells (12). Our results showed that TESC high expression positively correlated with TDS of DTC. TESC knockdown significantly inhibited ERK1/2 phosphorylation and decreased NIS expression in DTC cells, with a remarkably increased iodine uptake rate.
Interestingly, recovery of iodine uptake and NIS expression of IHH-4 was more sensitive than TPC-1 (Fig. 3), suggesting that TESC may have played a more important role in NIS inhibition caused by BRAF V600E mutation. Studies have shown that high TESC expression promotes primary megakaryocyte proliferation and differentiation (7). LncRNA ROR regulated TESC/ALDH1A1/TUBB3/PTEN axis to promote the PTC development and progression (13). Thus, TESC may be associated with DTC dedifferentiation through negative regulation of NIS and iodine uptake, particularly in BRAF V600E mutation DTC.
However, this study still has several limitations. First, we have not yet identified the specific downstream molecules of TESC involved in ERK1/2 activation contributing inhibition of NIS expression and iodine uptake, particularly under the premise of BRAF V600E mutation. Second, a further study about the specific mechanism and pathway of TESC deregulation on DTC dedifferentiation is still needed.
In conclusion, our study revealed that TESC was highly expressed in DTC, positively correlated with BRAF V600E mutation and clinicopathological features of metastasis, and promoted the growth and metastasis of DTC cells in vitro. Mechanistically, TESC promoted metastasis through EMT and induced iodine resistance by downregulating NIS in DTC cells. These results underscored the importance of TESC in DTC development and progression, providing a reliable biomarker for DTC diagnosis and prognosis and a potential oncogene target to treat DTC. Furthermore, we may perform in vivo experiments on nude mice to verify the oncogenic role of TESC in DTC.